They are fast, and have decent, twin linked firepower that's as strong as most anything in the codex.
Being so zippy it's easy to zoom around for side and rear armour shots, which actually might hit due to the twin-linked.
I run mine as singletons, so the squadron rules don't apply. Most folk don't find an evil go-kart too threatening, and ignore it in favour of the battlewagon nearby. I've killed many vehilces that made the mistake of ignoring buggies over the years.
They are low enough to gain cover from grots, but the gun is high enough to shoot over them with easy, and buggies (especially single ones) are very easy to hide or gain cover with.
Also, and most importantly, they are dirt cheap for what you get, and can shrug off most small arms fire (barring massive quantities or especially powerful guns (Tau rifles for example) ).
Not everyone finds them to be their thing. I like them, but i also use Deffkoptas and Stormboyz in my lists just as readily.
*edit for autocorrect software sucking dirt...

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